1. Field of the Invention
The present invention relates to a multilayered foam heat-shrinkable tube which easily adheres to and covers the surface of an article such as a metal pipe, a metal rod, or the like, and which provides good heat and electrical insulation, corrosion resistance, water resistance, and so on, to thereby protect the article. The invention also relates to a method for producing the multi-layer foam heat-shrinkable tube.
2. Description of the Related Art
A macromolecule foam material has been used for coating a surface of various articles such as pipes, rods, and the like, which provides heat and electrical insulation, corrosion resistance, water resistance, cushion, and the like.
One conventional method of coating the surface of an article with the macromolecule foam material is to extrude the macromolecule directly onto the outer surface of an article. Another conventional method is to form a foam pipe having an inner diameter equal to an outer diameter of a metal pipe and thereafter to insert the metal pipe into the foam pipe. However, in this method, it is necessary to form foam pipes having different inner diameters corresponding to metal pipes having different outer diameters. Further, it is impossible to insert a metal pipe having an outer diameter veried in the middle of the pipe into a foam pipe. Thus, this method is troublesome and unreasonable.
Japanese Patent Publication No. Sho-58-3819 discloses a foam heat-shrinkable tube obtained by extrusion-molding a thermoplastic polymer containing a foam agent. An extrusion-molded foam tube is softened by heat and the inner diameter is expanded. Then, the tube is cooled while the inner diameter at the expanded state is maintained, so that the foam heat-shrinkable tube is formed. After being applied onto an article, such as a metal pipe or the like, so as to cover it, the tube is heated which allows the tube to shrink to the inner diameter at the extruded state and to foam. However, foaming of the tube decreases the adhesion between the tube and the article because the inner diameter of the tube increases by the foaming so that a gap is generated between the tube and the article. Further, heating the tube by a heat gun or the like may cause uneven foaming of the tube.
Japanese Utility Model Unexamined Publication No. Sho-59-34195 discloses a heat-shrinkable tube having an inner surface on which a heat-insulating material such as a macromolecule foam body or the like is provided. However, since the heat-insulating material has no heat shrinking properties, the adhesion between the heat-insulating material and the surface of the article is unsatisfactory.
If the adhesion between the foam layer and the surface of the article is unsatisfactory, water, such as rain water or the like, enters between the foam layer and the article to thereby corrode the article.
A foam heat-shrinkable tube made of a thermoplastic polymer that has finished foaming can be made by heating and expanding the inner diameter of the tube. When the tube is applied onto an article, such as a metal pipe or the like, and then heated to shrink, good adhesion between the tube and the article can be formed.
However, if the foaming ratio, which is defined as a ratio of specific gravity of a resin before foaming to that of the resin after foaming, of the foam layer is made high to improve its heat-insulating capacity, its density is so low that its mechanical strength, such as tensile strength or the like, is insufficient. If the foaming ratio is made low to increase the mechanical strength, the heat insulating capacity or the like, which is an original characteristic of the foam tube, is insufficient.
Further, to attain high corrosion resistance, the adhesion between the tube and the surface of an article must be improved.